The invention relates to a security system to prevent unauthorized starting of the engine of a vehicle, with a remote control unit containing a transponder which, on reception of an interrogation signal by means of at least one reception aerial, transmits an identification code group by means of a transmitter unit, and a control unit located in the vehicle which, on actuation of a starter button within the vehicle, transmits the interrogation signal, and which subsequently checks the identification code group transmitted by the remote control unit and enables the starting procedure of the engine only when the identification code group coincides with the code group expected by it.
Modern motor vehicles are equipped with electronic security systems that prevent unauthorised persons from opening the motor vehicle and starting the engine. In order to open the vehicle and to start the engine, authorised persons must carry a remote control unit in which is stored an identification code group which is transmitted by the remote control unit and which can be checked by a control unit within the vehicle, whereby the control unit in the vehicle will allow access to the vehicle only when this identification code group coincides with the code group expected by it.
Such a security system, by means of which only authorised persons are allowed access to the vehicle, is described in patent application DE 100 12 637.5.
In the case of this security system, the control unit in the vehicle transmits an interrogation signal as soon as the person wanting access to the vehicle touches the door handle. The remote control unit, which is carried by the person and which may be located, for example, in the car key, in a key tag or even in a so-called chip card, receives this interrogation signal and subsequently re-transmits the identification code group to the control unit within the vehicle. The control unit then checks this code group for coincidence with the code group expected by it and, on positive verification of coincidence, allows access to the vehicle. The person who touched the door handle can therefore operate the door handle as if the vehicle had not been locked. This is because the interchange of signals between the control unit and the remote control unit takes place in such a short time that no delay is felt during the mechanical opening process of the vehicle. This security system can be complemented by a further security system, or it can be combined with an already existing security system, which only allows the engine to be started after the execution of an individual verification process and positive confirmation. Such an enhanced security system is especially advantageous when the engine is not started by introducing an ignition key into a specifically provided ignition lock, but simply by pressing a starter button. Without the provision of additional security measures against unauthorised starting of the engine of a vehicle equipped in this way, dangerous situations may easily, arise. Assuming that an authorised person has opened the vehicle door, the engine could be started by pressing the starter button without any further security check taking place. If, for example, a child, being in the vehicle after the door has been opened, pressed the starter button, then the engine would start whilst the person intending to carry out the starting process is not yet in the vehicle. The vehicle could thus move off and cause a dangerous situation.
The invention therefore rests on the requirement of creating a security system of the type described above, whereby any unauthorised starting of the engine is prevented, even if the vehicle has been opened by an authorised person after completion of a first security check.
With a security system of the type described above, this objective is achieved by a signal detector, connected to the reception aerial, whose reaction threshold can be toggled between a high and a low value, and which relays the interrogation signal for further evaluation only when the interrogation signal level exceeds the set reaction threshold, a code verification circuit, connected to the signal detector, which checks the interrogation signal relayed by it as to whether it contains a pre-established code group and, on recognition of this code group, switches the reaction threshold of the signal detector to its high value, whereby the code verification circuit will cause the transmitter unit to transmit the identification code group only when it receives the interrogation signal, even after the signal detector has switched over to its high reaction threshold level.
The security system according to the invention not only checks whether the person who pressed the starter button of the vehicle is authorised to start the engine of the corresponding vehicle, but even before this check, it has already verified whether this person is inside the vehicle and in the area of the driving seat. By increasing the reaction threshold level of the signal detector after reception of the interrogation signal at a low reaction threshold level, it can be positively asserted that the person carrying the remote control unit is in the immediate proximity of the starter button, that is in the interior of the vehicle, when the signal is still received even at the high reaction threshold level. This makes it impossible for the dangerous situation to arise where the engine is started by a person inside the vehicle whilst the person carrying the remote control unit is still outside the vehicle.
In an advantageous embodiment, the security system according to the invention comprises two further reception aerials which are spatially arranged in such a way that the main direction of reception of each of the three reception aerials is aligned to a different one of each of the three mutually perpendicular spatial axes, and that each of the two further reception aerials is connected to a signal detector of the same type as the one connected to the first reception aerial. Because of the use of a total of three reception aerials, a very good spatial acquisition of the electromagnetic field originated by the interrogation signal is achieved, which improves on the certainty of the assertion that the person carrying the remote control unit is, in fact, inside the vehicle and actually in the driving seat.
It is furthermore advantageous to provide for the reaction threshold levels of the signal detectors to be adjustable in accordance with the spatial conditions of the intended application field of the remote control unit. This setting of the reaction threshold levels is carried out once during the production process of the remote control unit, to suit the conditions in the vehicle to which the remote control unit will be assigned. This refinement also contributes to improving the certainty of locating the position of the person carrying the remote control unit.